In the high-production cookware, appliance and automotive industries, as well as the low- and medium-production aircraft, aerospace, and job-shop industries, metallic sheet may be formed by a variety of different dies, the type and size of the die being dictated by the shape and intended use of the particular part. One process which is used to form a wide variety of these parts is the conventional drawing process. In a draw die, the blank is drawn across a binder surface allowing metal to flow from the bind surface and onto the part. Unfortunately, variable and non-uniform stresses are thereby developed throughout the part which results in localized stretching. This creates severe springback and shape retention problems which makes it nearly impossible to predict, especially with large parts, the amount of springback that will occur. The common practice to overcome this springback or shape retention problem is to overcrown (deform beyond the desired shape) the part. Finding the appropriate degree of overcrown requires a number of costly trial and error procedures. There is also a significant amount of material waste in the drawing process because the blank is oversized to compensate for the metal flowing across the binder surface and to account for varying part strength resulting from non-uniform work hardening.
In my U.S. Pat. No. 4,576,030, I describe a process wherein sheet metal can be one hundred percent stretch formed between co-acting male and female die halves. This is accomplished by providing a pair of opposed gripper steels, at least one of which is provided with a number of spaced apart beads adapted to bite into the sheet metal, around the periphery thereof, when the gripper steels are closed. This permits the sheet metal to be homogeneously, one hundred percent stretch formed, thus resulting in a higher quality of shape retention, a reduction in the number of shock lines and stretch lines, less waste, and increased overall part strength.
Another procedure which enhances the quality of the formed part is that of fluid forming, that is, applying pressurized fluid against one side of the blank in the forming process. The benefits include increased versatility, a better finish on the final part, and reduced tool maintenance costs.
While all these advancements have continued to improve the quality of the part and to stretch the limits of product design, the dies and the supporting machinery and hardware have become larger, more diverse and more expensive. Furthermore, the competitive market dictates a continuous stream of operationally improved and aesthetically novel products. Each new product requires new parts which require new dies, supporting machines and hardware to produce them. Aside from the obvious economic strains associated with repeated design and testing of a new product, the time it takes to transform a part from concept to reality, often measured in years, has a discouraging effect on potential innovation.
What is desired is a sheet forming apparatus that combines the favorable aspects of fluid forming with the advantages of one hundred percent stretch forming; that permits a more accurate approximation of the desired part, reducing if not eliminating the prototype and testing procedure; that can be retooled more easily and more cheaply than existing assemblies; and that is adaptable for operation in conventional, standard sized presses.